It has been conventionally known an audio signal processing system having an automatic compensation function of an input signal as described in PTL1.
In the system described in PTL1, in a signal input device, an input port inputting an analog audio signal is provided with an analog amplifier for adjusting a level of an input signal, an AD converter which analog-digital converts an output of the analog amplifier, a compensation unit with a digital amplifier adjusting a level of an output signal of the AD converter, and a switch unit selecting either of the output signal of the AD converter and an output signal of the compensation unit, and a signal selected by the switch unit is outputted to a subsequent stage.
Further, on a console, a first control for accepting an on/off operation of automatic compensation and a second control for accepting a change operation of gain of the analog amplifier are provided corresponding to the input port.
Then, when a user operates the first control, the signal input device switches on and off of automatic compensation of the corresponding input port. Then, the switch unit selects the output signal of the compensation unit when the automatic compensation is on, or selects the output signal of the AD converter when it is off.
Further, when the user operates the second control, the signal input device changes gain of the analog amplifier at the corresponding input port according to the operation amount thereof. Further, when automatic compensation is on, gain of the compensation unit is changed by an amount that cancels out the change in gain of the analog amplifier.
Therefore, when automatic compensation is off, the signal input device outputs to the subsequent stage a signal adjusted in level according to gain set by operating the second control. On the other hand, when automatic compensation is on, the amount of change is canceled out by the compensation unit even when setting of gain is changed by operation of the second control, and thus a signal adjusted in level by substantially constant gain is outputted to the subsequent stage even when the second control is operated.